Energy filter for charged particle beam apparatus

ABSTRACT

This invention provides a method for improving performance of a reflective type energy filter for a charged particle beam, which employs a beam-adjusting lens on an entrance side of a potential barrier of the energy filter to make the charged particle beam become a substantially parallel beam to be incident onto the potential barrier. The method makes the energy filter have both a fine energy-discrimination power over a large emission angle spread and a high uniformity of energy-discrimination powers over a large FOV. A LVSEM using this method in the energy filter can obviously improve image contrast. The invention also provides multiple energy-discrimination detection devices formed by using the advantages of the method.

CLAIM OF PRIORITY

This application claims the benefit of priority of U.S. provisionalapplication No. 61/823,042 entitled to inventors filed May 14, 2013 andentitled “Energy Filter for Charged Particle Beam Apparatus”, the entiredisclosures of which are incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to a charged particle beam device, and moreparticularly to an energy filter for filtering a charged particle beamin terms of particle energies thereof. The invention also relates to acharged beam apparatus suitable for using such a device, such as anenergy-discrimination detection device in a low-voltage scanningelectron microscope (LVSEM) for inspecting defects on surfaces of wafersor masks in semiconductor manufacturing industry. However, it would berecognized that the invention has a much broader range of applicability.

BACKGROUND OF THE INVENTION

In the field of LVSEM and the related industrial fields which employ theprinciple of LVSEM to observe features on a specimen surface, such asdefect review and defect inspection of wafers or masks for yieldmanagement in semiconductor manufacture, forming an image of a specimensurface with high contrast and high resolution for the interestedfeatures has been demanded and pursued.

In a LVSEM, a primary electron (PE) beam is generated by an electronsource, and then focused onto and forced to scan the examined specimensurface by electron optics. The electron optics generally comprises acondenser lens, an objective lens and a deflector. The PE beam interactswith the specimen and make the specimen release a secondary emissionbeam therefrom. The secondary emission beam comprises secondaryelectrons (energy≦50 eV) and backscattered electrons (50<eV≦energy PEenergy). To limit both the radiation damage on the specimen and the PEbeam/specimen interaction to a very small volume beneath the specimensurface, the PE beam is designed to land on the specimen with low energy(<5 keV). In this case, the secondary electrons (SEs) and backscatteredelectrons (BSEs) are mainly related to the features on the specimensurface where the PE beam lands on, thereby becoming appropriate signalelectrons for forming images of the specimen surface. The SEs and/orBSEs are detected by detectors to obtain images of the examined specimensurface.

The contrast of the image in the LVSEM can be generated by many factorsand mainly depends on the detection of the signal electrons (SEs andBSEs). The yield δ of the secondary electron (SE) emission (ratio of thenumber of SEs and the number of primary electrons) changes with theincidence angle (relative to the specimen surface normal) of the PE beamand not sensitive to atomic numbers of the materials of the specimensurface, and therefore a SE image (obtained by detecting SEs) can showthe topography of the specimen surface due to geometric inclinationdifferences (topography contrast). The coefficient η of thebackscattered electron (BSE) emission (ratio of the number of BSEs andthe number of primary electrons) changes with atomic numbers of thematerials of the specimen surface and less sensitive to the localinclinations on the specimen surface, and as a result a BSE image(obtained by detecting BSEs) can show material differences (materialcontrast) of the specimen surface. Furthermore, a Low-Loss (LL) BSEimage (obtained by detecting BSEs with energy loss of around 100 eV orless) can exhibit a good contrast for nano-material composition withsubtle compositional variation. Besides, an SE image can show potentialdifferences (voltage contrast) on the specimen surface because SE yieldδ also changes with PE energy and SEs are too slow to sustain theinfluence of the additional electric field generated by the potentialdifferences.

Although the resolution of an image in the LVSEM is fundamentallydetermined by the aberrations of the electron optics thereof, howevermany factors can deteriorate it. An amount of net electrical chargesappears on the specimen surface if the total yield σ (=δ+η) of thesecondary emission is not equal to 1. As an SE image is easilyinfluenced by the charges, BSE images have become very important forhigh resolution investigation of critical or charging specimens.

In the LVSEM, both SEs and BSEs travel in substantially same directions.Therefore the signal electrons collected by an electron detector will bethe combination of SEs and BSEs, and the image may comprise topographycontrast, material contrast and voltage contrast due to conventionalelectron detectors have very low sensitivities to electron energies. Toclearly show some interested features, the image in an application maybe required to comprise only one kind of the contrasts. Accordingly, thedetection of the signal electrons (SEs and BSEs) is better sensitive toelectron energies; i.e. it is desired to use an energy-discriminationdetection which can select signal electrons in terms of energiesthereof. Typically, the energy-discrimination detection is realized bymaking the electrons pass through an energy filter before beingcollected by a conventional detector.

The intensity distributions of SEs and BSEs in the beam of secondaryemission depend on initial kinetic energies and emission angles thereof,and therefore the filtering function of the energy filter is preferredto be energy-depending other than energy-angle-depending. Theenergy-depending filtering is only sensitive to electron energies, andthe energy-angle-depending filtering is sensitive to electron emissionangles as well as electron energies. For the secondary emission by alow-energy PE beam, the angular distributions of SEs and BSEsrespectively conform Lambert's law (proportional to cos φ, where φ isemission angle relative to the surface normal of the specimen). Thelarge spread in emission angles makes it difficult to realize a pureenergy-depending filtering, and the real filtering function depends onelectron emission angle more or less. In addition, because of thedeflection effect of the deflector(s) and the geometric magnificationeffect of the objective lens on the trajectories of the SEs and BSEs,the incident situations of SEs and BSEs, when entering the energyfilter, change with the original locations on the field of view (FOV) ofthe specimen surface. For example, the beam of SEs from the FOV centeris incident onto the entrance of the energy filter along the opticalaxis thereof, while the beam of SEs from an edge point of the FOV has 2°incident angle and 3 mm off-axis shift (both relative to the opticalaxis). Consequently, the filtering function of the energy filteractually also depends on the original positions of electrons on the FOVmore or less, thereby being position-depending filtering to a certaindegree.

The available energy filters can be classified into two types,dispersion (axial or radial) type and reflection type. An energy filterof axial or radial dispersion type uses a dispersive element to makeelectrons with different energies generate different displacements inaxial or radial direction correspondingly, while an energy filter ofreflection type uses a potential barrier to reflect back the electronswith initial kinetic energies not higher than a specific value so as toprevent them from passing through. The dispersive element can be a lens(U.S. Pat. Nos. 7,544,937, 7,683,317) or a deflector (U.S. Pat. No.7,276,694), and the potential barrier can be an equipotential formed bya hollow electrode (U.S. Pat. No. 7,335,894) or a grid electrode (U.S.Pat. Nos. 7,141,791, 7,544,937, 7,683,317, 7,714,287, 8,203,119). Energyfilters of axial dispersion and reflection types are well usedindividually or in combination in the LVSEM due to being compact andsimple in configuration.

Accordingly, the filtering function of the energy filter can beevaluated by energy-discrimination power at the FOV center anduniformity of energy-discrimination powers over the entire FOV withinthe required range (such as 0.2 eV˜5 keV for a LVSEM) of the landingenergy of the PE beam. The energy-discrimination power for a point inthe FOV is the variation of energy thresholds with respect to theemission angle spread, while the uniformity of energy-discriminationpowers over the entire FOV is the variation of the energy thresholds ofthe chief rays (with 0° emission angles) coming from the entire FOV. Forthe energy filter of reflection type such as the energy filter 3 infront of the detector 7 in FIG. 1A, if the energy thresholds for 0°emission angle (the chief ray 2 c) and 45° emission angle (the marginray 2 m) of the SEs from the center point BO of the FOV on the surfaceof the specimen 4 are 2 eV and 2.1 eV respectively, theenergy-discrimination power is 0.1 eV with respect to 45° emission anglespread for the FOV center. If the energy thresholds of the chief raysover the entire FOV of 50 um×50 um square are within the range of 2 eV˜3eV, the uniformity of the energy-discrimination powers over the entireFOV is equal to the range, i.e., 1 eV. The lower the variation of theenergy thresholds for the FOV center is, the fine theenergy-discrimination power will be; while the lower the range of theenergy thresholds of the chief rays over the FOV covers, the higher theuniformity of the energy-discrimination powers will become.

Energy filters of reflection type can be placed near the specimensurface or the detector, as shown in FIGS. 1B and 1C. In both FIGS. 1Band 1C, the PE beam 1 is finally focused by the objective lens 5 andlands on the specimen 4. The electron beam 2 of secondary emission isemitted from where the specimen 4 is excited by the PE beam 1, whichcomprises SEs such as the SEs 2_1 and BSEs such as the BSEs 2_2. In FIG.1B, the hollow electrode 6 is negatively biased with respect to thespecimen 4 to form a potential barrier PB1 therebetween, and the SEs 2_1with initial kinetic energies not higher than the energy thresholds withrespect to the potential barrier PB 1 are therefore reflected back tothe specimen 4 and the other SEs and BSEs can pass the potential barrierand can be detected by the detector 7. This method can not be used to anapplication requiring a strong extraction field on the specimen surfaceto make more electrons escape from specimen surface or get highresolution. In FIG. 1C, the grid electrode 8 is negatively biased withrespect to the specimen 4 and becomes a potential barrier PB2 itself,and SEs 2_1 with energies not higher than the energy thresholds withrespect to the potential barrier PB2 are reflected back.

For an energy filter of reflection type, if the electrons which comefrom the FOV center with same energies and different emission angles canapproach the potential barrier with substantially equal angles ofincidence (relative to the correspondingly local normal of the potentialbarrier) such as shown in FIGS. 2A and 2B, the energy-discriminationpower will become finer. Meanwhile, if the electrons of the chief rayswhich come from the entire FOV with same energies can also approach thepotential barrier with substantially equal angles of incidence, theuniformity of the energy-discrimination powers over the FOV will becomehigher. If the potential barrier has a shape of sphere (corresponding toa focused beam) or plane (corresponding to a parallel beam), it ispossible to meet the foregoing requirements. In the light of thepotential barrier shape, the potential barrier formed by a hollowelectrode (such as in U.S. Pat. No. 7,335,894) has a shape ofhyperboloid and therefore is not advantageous than the potential barrierformed by a flat grid electrode in getting a fine energy-discriminationpower. Furthermore, although the hollow electrode incurs no electronloss due to lack of the electrons hitting on the wires of the gridelectrode, the dramatically deteriorated energy-discrimination power dueto the strong energy-angle-depending filtering and position-dependingfiltering puts a limitation onto the acceptable emission angle spreadand the acceptable size of the FOV, thereby limiting the intensity ofthe image signal and the throughput of the application.

Among the foregoing patents using a grid electrode to form a potentialbarrier, some (U.S. Pat. Nos. 7,683,317 and 8,203,119) assume thesecondary emission beam is parallel when entering the energy filter andthereafter attempt to keep it parallel and normally incident onto thepotential barrier, some (U.S. Pat. Nos. 7,141,791 and 7,714,287) adjustthe incident direction of the secondary emission beam with respect tothe position thereof in the FOV before entering the energy filter, andsome (U.S. Pat. No. 7,544,937) makes the grid electrode with a specialshape to fit the secondary emission beam. The foregoing cases either canonly work well within a small emission angle spread and a small FOV, orneeds at least an additional element for specially adjusting thesecondary emission beam before entering the energy filter or a complexgrid. To reduce the emission angle spread to fit the energy filter, alot of electrons with large emission angles in the secondary emissionbeam have to be cut off, thereby reducing the number of the detectedsignal electrons. To get a stronger image signal, the scanning speed hasto be slow down to increase the integration time of each pixel in theimage. The small FOV corresponds to a low throughput of observationbecause more moving steps are required to observe a large area on thespecimen surface. The additional space is thus needed to accommodate theadditional adjusting element(s), thereby making the entire apparatusbulky. Apparently, in the available energy filters of reflection type,there is no means for directly improving the incident situation of asecondary emission beam on the potential barrier if the secondaryemission beam is not parallel when entering the energy filter.

Accordingly, an energy filter for energy-discrimination detection in aLVSEM, which can provide a fine energy-discrimination power within alarge emission angle spread and a high uniformity ofenergy-discrimination power over a large FOV, is needed. Such an energyfilter will be more advantageous to improve the image contrast than theprior of art.

BRIEF SUMMARY OF THE INVENTION

The object of this invention is to provide an energy filter ofreflection type to realize energy-discrimination detection in a chargedparticle apparatus. By specifically providing a means inside the energyfilter to directly adjust the incident situation of a charged particlebeam onto a potential barrier of the energy filter, the energy filtercan provide a fine energy-discrimination power at a center of a FOV anda high uniformity of energy-discrimination powers over the entire FOV.Furthermore, by specifically arranging detectors associated with theenergy filter to separately detect SEs and BSEs within different energyranges, images respectively comprising topography contrast and/orvoltage contrast, and material contrast can be obtained simultaneously.Hence, this invention provides an effective way to realizeenergy-discrimination detection which can provide multiple images withhigh contrast and high resolution in a LVSEM and the related apparatusesbased on LVSEM principle, such as the defect inspection and defectreview in semiconductor yield management.

Accordingly, the invention provides a means to improve the filteringfunction of an energy filter of reflection type. The means uses abeam-adjusting lens on the entrance side of the potential barrier of theenergy filter to make an incident charged particle beam become asubstantially parallel beam to be incident onto the potential barrier.

The invention therefore provides an energy filter to filter a chargedparticle beam, which comprises a grid electrode being set at a firstpotential to form a potential barrier, and a beam-adjusting lens beingexcited to make the charged particle beam become a substantiallyparallel beam to be incident onto the potential barrier. A firstplurality of particles of the charged particle beam, which has initialkinetic energies higher than a specific value and thus is able to crossthe potential barrier, passes through the grid electrode and forms anexiting beam, while a second plurality of particles of the chargedparticle beam, which has initial kinetic energies not higher enough tobe able to cross the potential barrier, is reflected back from the gridelectrode and forms a reflection beam. An optical axis of thebeam-limiting lens is an optical axis of the energy filter.

In the energy filter, the grid electrode is perpendicular to and alignedwith the optical axis of the beam-limiting lens. The charged particlebeam can enter the energy filter along the optical axis thereof. Thecharged particle beam also can enter the energy filter with an angle anda radial shift both with respect to the optical axis thereof. Thebeam-adjusting lens is an electrostatic lens which comprises a firstelectrode, a second electrode and a third electrode. With respect to thecharged particle beam, the first electrode and the second electrode arerespectively on an entrance side and an exit side of the beam-adjustinglens and the third electrode is between the first electrode and thesecond electrode. The grid electrode is inside the second electrode. Thesecond electrode is set at the first potential, the first electrode isset at a potential of a neighborhood on the entrance side of thebeam-adjusting lens, and a potential of the third electrode is adjustedto make the charged particle beam become a substantially parallel beamto be incident onto the potential barrier.

The invention therefore provides an energy-discrimination detectiondevice for detecting a charged particle beam, which comprises an energyfilter and a first detector. The energy filter comprises a gridelectrode being set at a first potential to form a potential barrier,and a beam-adjusting lens below the grid electrode and being excited tomake the charged particle beam become a substantially parallel beam tobe incident onto the potential barrier. A first plurality of particlesof the charged particle beam, which has initial kinetic energies higherthan a specific value and thus is able to cross the potential barrier,passes through the grid electrode and forms an exiting beam, while asecond plurality of particles of the charged particle beam, which hasinitial kinetic energies not higher enough to be able to cross thepotential barrier, is reflected back from the grid electrode and forms areflection beam. The first detector, above the energy filter, is excitedto detect charged particles of the exiting beam. An optical axis of thebeam-limiting lens is both an optical axis of the energy filter and anoptical axis of the energy-discrimination detection device.

In the energy-discrimination detection device, the grid electrode isperpendicular to and aligned with the optical axis of the beam-adjustinglens. The energy-discrimination detection device may further comprise ashielding box covering the energy filter and the first detector. Theshielding box is made of electric conductor material and has an entranceplate which is below the energy filter, perpendicular to the opticalaxis thereof and has an entrance grid for the charged particle beampassing through. The shielding box can be set at a potential of aneighborhood where the energy-discrimination detection device is placed.

The energy-discrimination detection detector may further comprise abeam-focusing lens between the first detector and the energy filter,wherein the beam-focusing lens is excited to reduce a beam size of theexiting beam on the first detector. The energy-discrimination detectiondevice may further comprise a second detector below the entrance grid,wherein the second detector has an opening for a central part of thecharged particle beam passing through and fully or partially detectsother part thereof. The energy-discrimination detection device mayfurther comprise a third detector being placed and excited to detect thereflection beam.

The energy-discrimination detection device may further comprise a firstaperture plate below the entrance grid. The first aperture plate has aplurality of apertures with different radial sizes and one of theplurality of apertures is selected to block a peripheral part of thecharged particle beam before passing through the entrance grid. Theenergy-discrimination detection device may further comprise a seconddetector below the first aperture plate, wherein the second detector hasan opening for a central part of the charged particle beam passingthrough and fully or partially detects other part thereof. Theenergy-discrimination detection device may further comprise a secondaperture plate below the second detector. The second aperture plate hasa plurality of apertures with different radial sizes and one of theplurality of apertures is selected to block a peripheral part of thecharged particle beam before going to the second detector. Theenergy-discrimination detection device may further comprise a thirddetector being placed and excited to detect the reflection beam.

The invention further provides an electron beam apparatus for observinga surface of a specimen, which comprises an electron source beingexcited to emit primary electrons along an optical axis of the electronbeam apparatus, an accelerating electrode below the electron source andhaving an opening aligned with the optical axis for primary electronspassing through, a condenser lens below the accelerating electrode andaligned with the optical axis, a beam-limiting aperture plate below thecondenser lens and having a plurality of apertures with different radialsizes, a magnetic objective lens below the beam-limiting aperture plateand aligned with the optical axis, a retarding electrode below themagnetic objective lens and having an opening aligned with the opticalaxis for the primary electron beam passing through, a specimen stagebelow the retarding electrode and supporting the specimen, a deflectionunit between the beam-limiting aperture plate and the retardingelectrode, an energy-discrimination detection device above thedeflection unit and away from the optical axis and a Wien filter betweenthe specimen and the energy-discrimination detection device.

The accelerating electrode is excited to accelerate primary electrons tohave desired first energies. One of the plurality of apertures of thebeam-limiting aperture plate is selected as a beam-limit aperture andthus aligned with the optical axis. The condenser lens is excited tomake a part of the primary electrons pass through the beam-limitaperture so as to form a primary electron beam with a desired currentvalue. The specimen surface is opposite to the retarding electrode, andboth specimen and retarding electrode are excited to decelerate primaryelectrons of the primary electron beam to land on the specimen surfacewith desired second energies much lower than the first energies. Themagnetic objective lens is excited to focus the primary electron beam toform a focused probe on the specimen surface and the focused probereleases a secondary emission beam therefrom which comprises secondaryelectrons and backscattered electrons. The deflection unit deflects theprimary electron beam and thus makes the focused probe scan the specimensurface.

The energy-discrimination detection device includes an energy filter anda first detector. The energy filter comprises a grid electrode and abeam-adjusting lens. The grid electrode functions as a potentialbarrier, and the beam-adjusting lens is below the grid electrode andfunctions as a means for adjusting an incident electron beam of theenergy filter to become a substantially parallel beam to be incidentonto the potential barrier. A first plurality a part of electrons of theincident electron beam, which has initial kinetic energies higher than aspecific value and thus is able to cross the potential barrier, passesthrough the grid electrode and forms an exiting beam, while a secondplurality of electrons of the incident electron beam, which has initialkinetic energies not higher enough to be able to cross the potentialbarrier, is reflected back from the grid electrode and forms areflection beam. The first detector is above the energy filter to detectexiting beam of the energy filter.

The Wien filter can be excited to deflect the secondary emission beam tobe incident onto the energy-discrimination detection device while notdeflecting the primary electron beam, and therefore the secondaryemission beam is the incident electron beam of the energy filter. Theelectron beam apparatus further comprises a second detector above thedeflection unit and away from the optical axis, and the Wien filter canbe excited to deflect the secondary emission beam to be detected by thesecond detector while not deflecting the primary electron beam.

The invention also provides a method for improving performance of anenergy filter of reflection type for a charged particle beam, whichcomprises a step of providing a beam-adjusting lens on an entrance sideof a potential barrier of the energy filter, wherein the beam-adjustinglens is excited to make the charged particle beam become a substantiallyparallel beam to be incident onto the potential barrier.

Other advantages of the present invention will become apparent from thefollowing description taken in conjunction with the accompanyingdrawings wherein are set forth, by way of illustration and example,certain embodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be readily understood by the followingdetailed description in conjunction with the accompanying drawings,wherein the same or like reference numerals designate the same or likestructural elements, and in which:

FIGS. 1A˜1C are schematic illustrations of energy-discriminationdetection in a LVSEM;

FIGS. 2A and 2B are schematic illustrations of incident situations of asecondary emission beam approaching a potential barrier inside an energyfilter;

FIGS. 3A and 3B are schematic illustrations of a fundamentalconfiguration of an energy filter in an energy discrimination detectiondevice for a charged particle beam with the normal incidence and anoblique incidence respectively in accordance with one embodiment of thepresent invention;

FIG. 4 is a schematic illustration of an energy filter with anelectrostatic lens in accordance with one embodiment of the presentinvention;

FIGS. 5A˜5D show simulation results of beam trajectories andequipotentials of a secondary emission beam in a LVSEM and beingnormally incident into an energy filter with a structure as shown inFIG. 4;

FIGS. 6A and 6B show simulation results of beam trajectories andequipotentials of a secondary emission beam in a LVSEM and beingobliquely incident into the energy filter shown in FIGS. 5A˜5D;

FIG. 7 is a schematic illustration of a configuration of anenergy-discrimination detection device with an entrance beam-limitaperture and an energy filter as shown in FIG. 3A in accordance with oneembodiment of the present invention;

FIG. 8 is a schematic illustration of a configuration of an energydiscrimination detection device with a beam-focusing lens and an energyfilter as shown in FIG. 3A in accordance with one embodiment of thepresent invention;

FIGS. 9A˜9C are respectively a schematic illustration of a configurationof an energy-discrimination detection device with an energy filter asshown in FIG. 3A and providing more than one kind image signals inaccordance with one embodiment of the present invention;

FIGS. 10A and 10B are respectively a schematic illustration of aconfiguration of energy-discrimination detection device with an energyfilter as shown in FIG. 3A, which has an entrance beam-limit andprovides more than one kind image signals in accordance with oneembodiment of the present invention;

FIG. 11 is a schematic illustration of a LVSEM with anenergy-discrimination detection device and a conventional detectiondevice in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Various example embodiments of the present invention will now bedescribed more fully with reference to the accompanying drawings inwhich some example embodiments of the invention are shown. Withoutlimiting the scope of the protection of the present invention, all thedescription and drawings of the embodiments will exemplarily be referredto an electron beam. However, the embodiments are not be used to limitthe present invention to specific charged particles.

In the drawings, relative dimensions of each component and among everycomponent may be exaggerated for clarity. Within the followingdescription of the drawings the same or like reference numbers refer tothe same or like components or entities, and only the differences withrespect to the individual embodiments are described.

Accordingly, while example embodiments of the invention are capable ofvarious modifications and alternative forms, embodiments thereof areshown by way of example in the drawings and will herein be described indetail. It should be understood, however, that there is no intent tolimit example embodiments of the invention to the particular formsdisclosed, but on the contrary, example embodiments of the invention areto cover all modifications, equivalents, and alternatives falling withinthe scope of the invention.

In this invention, “axial” means “in the optical axis direction of anapparatus, column or a device such as a lens”, while “radial” means “ina direction perpendicular to the optical axis”.

Next, the present invention at first provides an energy filter ofreflection type and then combines the energy filter with conventionaldetectors and beam-limit means to construct basic, or advancedenergy-discrimination detection devices for a LVSEM.

As well known, an energy filter of reflection type is based on the lawof conservation and conversion of energy of a charged particle. For anelectron, its total energy is the sum of kinetic energy and potentialenergy thereof. When the electron moves from an initial place on anequipotential P1 to another place on an equipotential P2, its totalenergy keeps constant but a conversion between the kinetic energy andthe potential energy happens if the potentials V₁ and V₂ of the twoequipotentials P1 and P2 are different. The electron will return back ifits speed component in the normal direction of the equipotential P2(called as normal speed component on the equipotential P2) is reduced tozero when it arrives there. Therefore, to pass the equipotential P2, thekinetic energy of the electron at the initial place (called as initialkinetic energy hereinafter) needs to be higher than a specific valueE_(KT) to ensure a non-zero normal speed component. The specific valueE_(KT) is the threshold of initial kinetic energy (simply called asenergy threshold hereinafter) for an electron coming from theequipotential P1 to pass the equipotential P2. The energy thresholdE_(KT) depends on the potential difference of the equipotentials P1 andP2 and the incident situation of the electron on the equipotential P2,as shown in the equation (1) where v_(T) is the speed component of theelectron in the tangent direction of the equipotential P2 (called astangent speed component on the equipotential P2).E _(KT)=(e·V ₂ −e·V ₁)+½·m·v _(T) ²  (1)

For a certain value of the potential difference V₂−V₁, the energythreshold E_(KT) changes with the incident situation of the electron andhas the minimum value when the tangent speed component v_(T) is zero. Inthe case where the equipotential P2 is plane, apparently, if an electronbeam emitted from a place on the equipotential P1 is changed into aparallel beam to be incident onto the equipotential P2, the energythreshold E_(KT) will be the same within the emission angle spread ofthe electron beam. For an energy filter of reflection type used in anenergy-discrimination detection device in a LVSEM, analogously, thespecimen surface can be at the equipotential P1, the equipotential P2can be the potential barrier of the energy filter, and the beam can be asecondary emission beam emitted from a point on the specimen surface.Therefore, if there is a beam-adjusting means inside the energy filterand just in front of the potential barrier thereof, which can make thesecondary emission beam become a substantially parallel beam to beincident onto the potential barrier, the energy threshold E_(KT) will bealmost the same within the emission angle spread of the secondaryemission beam. The variation of the energy thresholds with respect tothe emission angle spread is the energy-discrimination power. The moreparallel the beam becomes, the smaller the variation of energythresholds will be and consequently the finer the energy-discriminationpower will be.

The invention therefore provides an energy filter of reflection typeused for energy-discrimination detection in a LVSEM, which basicallycomprises a grid electrode and a beam-adjusting lens. The grid electrodefunctions as the potential barrier and the beam-adjusting lens functionsas the beam-adjusting means to make an incident electron beam become aparallel beam to be incident onto the potential barrier. FIGS. 3A and 3Bshow a fundamental configuration of such an energy filter 3 in aconfiguration of an energy-discrimination detection device 71. The basicunits in the detection device 71 are the detector 7 at the potential V₃and the energy filter 3. The energy filter 3 comprises the gridelectrode 32 at the potential V₂ and the beam-adjusting lens 33. Thegrid electrode 32 is perpendicular to the optical axis 3A of thebeam-adjusting lens 33. The detection device 71 can be used inside orclose to the column of the LVSEM. To avoid the mutual influences of thedetection device 71 and the column due to the magnetic and/orelectrostatic fields generated by each of them, the detection device 71is covered by a shielding box 30 which has an entrance plate 30_1 withan entrance grid 31. The incident electron beam 2 is a secondaryemission beam from the center point of a FOV on a specimen surface atthe potential V₁, which enters the detection 71 by passing through theentrance grid 31. The shielding box 30 is made of electrical conductormaterial and at potential V₀ equal to the potential of the neighborhoodof the detection device 71. If there is at least one magnetic fieldinside and/or around the detection device 71, the shielding box 30 isrequired to be made of electric and magnetic conductor material.

In FIG. 3A, the incident electron beam 2 enters the energy filter 3along the optical axis 3A thereof. The potential V₂ of the gridelectrode 32 is set with respect to the potential V₁ and the desiredenergy threshold E_(KT) in terms of the equation (1) so as to form thedesired potential barrier. For example, if it is desired to reflect backthe electrons among the incident electron beam 2 and with initialkinetic energies not higher than 10 eV; i.e. the desired energythreshold E_(KT) is 10 eV, then V₂ can be set at −11510V whenV₁=−11500V. The beam-adjusting lens 33 is excited to make the incidentelectron beam 2 become a parallel beam to be normally incident onto thegrid electrode 32; in other words the electrons of the incident electronbeam are normally incident onto the grid electrode 32. The exitingelectron beam 2_P comprises the electrons with initial kinetic energieshigher than the desired energy threshold E_(KT) and the reflectionelectron beam 2_R comprises the other electrons.

In FIG. 3A, the reflection electron beam 2_R returns back almost alongthe path of the incident electron beam 2. For the LVSEM which has ameans (such as Wien Filter) to separate the two beams, the reflectionelectron beam 2_R has no influence on the LVSEM. For the LVSEM withoutsuch a means, the reflection electron beam 2_R may become a disturbancesource. In this case, it is better to make the reflection electron beams2_R return back along a path separated from the path of the incidentbeam 2. To do so, the incident electron beam 2 needs to be obliquelyincident onto the grid electrode 32, as shown in FIG. 3B. In FIG. 3B,the incident electron beam 2 enters the energy filter 3 with an angle βof incidence and an off-axis shift or radial shift (both are relative tothe optical axis 3A of the beam-adjusting lens 33). Because the angle βand the radial shift influence the tangent speed components of theelectrons on the grid electrode 32, i.e. the situations of obliqueincidence, the potential V₂ of the grid electrode 32 has to be setaccording to these factors as well as the potential V₁ of the specimensurface and the desired energy threshold E_(KT) so as to form thedesired potential barrier. Although it is difficult to get an analyticalexpression about the relationship, the appropriate value of thepotential V₂ for an application can be obtained by a model simulation orextracted experimentally from the resulting images. The beam-adjustinglens 33 is excited to make the incident electron beam 2 become aparallel beam to be obliquely incident onto the grid electrode 32. Theexiting electron beam 2_P comprises the electrons with initial kineticenergies higher than the desired energy threshold E_(KT) and thereflection electron beam 2_R comprises the other electrons. In thiscase, an angle is formed between the paths of the incident electron beam2 and the reflection electron beam 2_R, thereby separating the twobeams. The reflection electron beam 2_R hits the entrance plate 30_1 andthen is absorbed.

The beam-adjusting lens 33 in FIG. 3A can be an electrostatic lens or amagnetic lens. In the case while an electrostatic beam-adjusting lens isused, the electrostatic beam-adjusting lens and the grid electrodetogether form the retarding field in front of the grid electrode. Hence,the grid electrode is usually close to or even inside an exit electrodeof the electrostatic beam-adjusting lens, as shown in FIG. 4. In FIG. 4,the beam-adjusting lens 33 is an electrostatic lens which comprises theentrance electrode 33_1, the exit electrode 33_2 and the adjustingelectrode 33_3. The entrance electrode 33_1 is placed above the entranceplate 30_1 and set at the potential V₃₁, while the exit electrodes 33_2comprises the upper part 33_2U and the lower part 33_2L respectivelyabove and below the grid electrode 32 and at the potential V₃₂. Theadjusting electrode 33_3 is placed between the entrance electrode 33_1and the exit electrode 33_2 and set at the potential V₃₃. The potentialsV₃₁ and V₃₂ can be simply set to be equal to V₀ and V₂ respectively, andthus the potential V₃₃ is adjusted to make the incident electron beam 2become a parallel beam to be normally incident onto the grid electrode32.

FIGS. 5A˜5D show simulation results of an energy-discriminationdetection device with an energy filter as shown in FIG. 4. The innerdiameters of the electrodes 33-1˜33_3 are equal to the same value D. Thedistance L₁ between the entrance grid 31 and the grid electrode 32 isequal to 1.4 times of the value D, while the distance L₂ between thegrid electrode 32 and the detector 7 is equal to half of the value D.The incident electron beam 2 comes from the center point of a FOV on aspecimen surface in a LVSEM and enters the energy filter 3 along theoptical axis thereof. The electrons of the incident electron beam 2 haveemission angles within the range 0°˜±40° (± angles are with respect totwo emission cases having 180° difference in azimuth). The potential V₁of the specimen surface is −11500V and the potential V₀ of thesurrounding environment where the energy-discrimination detection deviceis located is 0V. The desired energy threshold is 10 eV, and thereforethe potential V₂ of the grid electrode 32 is set at −11510V.Accordingly, the entrance electrode 33_1 and the exit electrode 33_2 areset at 0V and −11510V respectively.

In FIG. 5A, the potential V₃₃ of the adjusting electrode 33_3 is setequal to the potential V₀ of the entrance grid 31, similar to thesituation that the beam-adjusting lens 33 is absent. The two electronsboth with 0° emission angle and respectively having 10.06 eV and 10.05eV initial kinetic energies approach the grid electrode 32 along thelower central line. The electron with 10.06 eV energy passes through thegrid electrode 32 and lands on the detector 7 along the upper centralline, while the electron with 10.05 eV energy is reflected back from thegrid electrode 32 and returns to the entrance grid 31 along the lowercentral line. The two electrons both with 40° emission angle andrespectively having 20.65 eV and 20.64 eV initial kinetic energiesapproach the grid electrode 32 along the lower right line close to thelower central line. The electron with 20.65 eV energy passes through thegrid electrode 32 and lands on the detector 7 along the upper rightline, while the electron with 20.64 eV energy is reflected back from thegrid electrode 32 and returns to the entrance grid 31 along the lowermost right line. The lines on the left side of the lower and uppercentral lines are the trajectories of two electrons both with −40°emission angle and having 20.65 eV and 20.64 eV initial kinetic energiesrespectively. Therefore, the energy thresholds with respect to 0° and±40° emission angles are 10.05 and 20.64 eV respectively, andconsequently the energy-discrimination power is 10.59 eV.

In FIG. 5B, the potential V₃₃ of the adjusting electrode 33_3 is set at−5577V so as to adjust the incident situation of the incident electronbeam 2 onto the grid electrode 32; i.e. the beam-adjusting lens 33 isactive. Due to the electrostatic field of the beam-adjusting lens 33,the incident electron beam 2 becomes a parallel beam and the electronsthereof are substantially normally incident onto the grid electrode 32.The paths of the incident electron beam 2 and the reflection electronbeam 2_R are almost overlapped and difficult to be distinguished. Inthis case, the energy thresholds with respect to 0° and ±40° emissionangles are 10.02 and 10.01 eV respectively, and consequently theenergy-discrimination power is 0.01 eV, which is much better than thatwhen the beam-adjusting lens off.

FIGS. 5C and 5D show the improvement of the uniformity ofenergy-discrimination powers over the entire FOV of 50 um×50 um squarewhen the beam-adjusting lens is off and active respectively, whereinemission angles of all the electrons in the incident electron beam 2Care equal to 0°. In FIG. 5C, the adjusting electrode 33_3 is set at 0V,similar to the situation that the beam-adjusting lens 33 is absent. Theupper and lower central lines show the trajectories of the two electronswith 0° emission angle in FIG. 5A. Two electrons approach the gridelectrode 32 along the lower right line close to the lower central line,which are both emitted from one of the two edge points on a diagonal ofthe FOV and with 19.34 eV and 19.33 eV initial kinetic energiesrespectively. The electron with 19.34 eV energy passes through the gridelectrode 32 and lands on the detector 7 along the upper right line,while the electron with 19.33 eV energy is reflected back from the gridelectrode 32 and returns to the entrance grid 31 along the lower mostright line. The lines on the left side of the lower and upper centrallines are the trajectories of two electrons both emitted from another ofthe two edge points on the diagonal of the FOV and with 19.34 eV and19.33 eV initial kinetic energies respectively. Therefore, the energythresholds with respect to the FOV center and the FOV edges are 10.05 eVand 19.33 eV respectively, and consequently the uniformity ofenergy-discrimination powers over the entire FOV is 9.28 eV.

In FIG. 5D, the potential V₃₃ of the adjusting electrode 33_3 is set at−5577V, same as that in FIG. 5B. Due to the electrostatic field of thebeam-adjusting lens 33, the incident electron beam 2C becomes asubstantially parallel beam and the electrons thereof are substantiallynormally incident onto the grid electrode 32. The paths of the incidentelectron beam 2C and the reflection electron beam 2C_R are almostoverlapped and difficult to be distinguished. In this case, the energythresholds with respect to the FOV center and the FOV edges are 10.02 eVand 10.03 eV respectively, and consequently the uniformity ofenergy-discrimination powers over the entire FOV is 0.01 eV, which ismuch better than that when the beam-adjusting lens off.

FIGS. 6A and 6B show simulation results when the incident electron beam2 obliquely enters the foregoing energy-discrimination detection devicewith 4.2° angle of incidence and the off-axis shift equal to half of thevalue D. In FIGS. 6A and 6B all the potentials are same as that in FIGS.5B and 5D except the potential V₂. The potential V₂ is set −11418.05V soas to realize the same desired energy threshold as that in FIGS. 5A˜5D.

In FIG. 6A, the incident electron beam 2 comes from the center point ofa FOV on a specimen surface in a LVSEM and includes the electrons with0°˜±20° emission angles. Due to the electrostatic field of thebeam-adjusting lens 33, the incident electron beam 2 becomes asubstantially parallel beam to be incident onto the grid electrode 32.The paths of the incident electron beam 2 and the reflection electronbeam 2_R are respectively on the right and left sides of the opticalaxis 3A. In this case, the energy thresholds with respect to 0°, 20° and−20° emission angles are 10.02, 10.62 eV and 12.45 respectively, andconsequently the energy-discrimination power is 2.43 eV with respect to0°˜±20° emission angles, which is much better than that when thebeam-adjusting lens off. When the adjusting electrode is set at 0V, theenergy-discrimination power already deteriorates to 35 eV with respectto 0°˜±10° emission angles.

In FIG. 6B, the incident electron beam 2C includes the electrons emittedfrom the entire FOV of 20 um×20 um square on the specimen surface andwith (remission angles. In this case, the energy thresholds with respectto the FOV center and two edge points on a diagonal of the FOV are 10.02eV, 10.06 eV and 12.22 eV respectively, and consequently the uniformityof energy-discrimination powers over the entire FOV is 2.2 eV, which ismuch better than that when the beam-adjusting lens off. When theadjusting electrode is set at 0V, the uniformity ofenergy-discrimination powers over the FOV of 10 um×10 um square alreadydegrades to 25.2 eV.

It is clear that the performance of the energy filter 3 is better forthe incident electron beam in normal incidence than in obliqueincidence. That is because the aberrations of the beam-adjusting lens 33become large when the incident electron beam passes it with a largeoff-axis shift and a large angle of incidence. The large aberrationsgenerate large differences among incident situations of the electronsonto the potential barrier. To reduce the aberrations, the structure ofthe beam-adjusting lens can be further optimized or comprise moreelectrodes.

The energy-discrimination detection device using the foregoing energyfilter 3 can be varied so as to have more functions to meet the specialneeds of some applications. Based on the energy-discrimination detectiondevice 71 in FIGS. 3A and 3B, next the present invention providesseveral means to construct energy-discrimination detection devices withdiversified functions.

If the interested features on a specimen surface are trenches with highaspect ratio (depth to width), for the secondary electrons generated onthe bottom of a deep trench, only those with small emission angles canescape from the trench. For the applications of this kind, the imagecontrast will be better if the detector only detects the electrons withsmall emission angles. Therefore it would be better if theenergy-discrimination detection device can choose the emission anglespread of the incident electron beam. In the energy-discriminationdetection device 72 in FIG. 7, the aperture plate 34 with a plurality ofapertures is placed below the entrance grid 31 and perpendicular to theoptical axis 3A. The apertures of the aperture plate 34 have differentradial sizes. One of the apertures is selected and aligned with opticalaxis 3A to limit the incident electron beam so that the incidentelectron beam finally enters the energy filter 3 with the desiredemission angle spread. The selected aperture is the beam-limitingaperture for the energy filter 3. The aperture plate 34 is made ofelectrostatic material and preferred to be set at the potential V₀.

For the application requiring a large emission angle spread and/or alarge FOV, the electron beam 2_P may have a lager beam size afterexiting the energy filter 3. Usually the detector 7 is positively biasedwith respect to the specimen surface to make the signal electronsimpinge thereon with high landing energies so as to get a high gain.Hence, there is an accelerating field between the energy filter 3 andthe detector 7, which will focus the electron beam 2_P to a certaindegree. Because the detection area of a detector cannot be too large dueto the limitation of electric respond characteristics, the beam size ofthe electron beam 2_P may be still larger than the detection area of thedetector 7. In this case, it would be better if theenergy-discrimination detection device can adjust the size of theexiting electron beam on the detector. In the energy-discriminationdetection device 73 in FIG. 8, a beam-focusing lens 35 is placed betweenthe energy filter 3 and the detector 7. The beam-focusing lens 35 isexcited to reduce the beam size of the exiting electron beam 2_P on thedetector 7. The beam-focusing lens 35 can be an electrostatic lens or amagnetic lens. Please return back to FIG. 4, wherein the upper part33_2U of the exit boundary electrode 33_2 actually acts as a focusinglens to a certain degree. The focusing function is obvious on FIGS.5A˜5D.

For the application which requires to simultaneously provide multipleimages (forming by different signal electrons) of interested features ona specimen surface, the energy-discrimination detection device needs oneor more additional detectors. To additionally get dark-field image(formed by signal electrons with large emission angles) in theenergy-discrimination detection device 74 shown in FIG. 9A, onedark-field detector 71 is additionally placed below the entrance grid 31to detect the peripheral part of the incident electron beam 2 whichcomprises electrons with large emission angles. The dark-field detector71 has an opening for the center part of the incident electron beam 2passing through. To additionally get the SE image (formed by the signalelectrons with low initial kinetic energies) in theenergy-discrimination detection device 75 shown in FIG. 9B, the incidentelectron beam 2 is obliquely incident onto the energy filter 3 and oneSE detector 72 is placed on the path of the reflection electron beam 2_Rto detect the electrons thereof. Apparently, the energy-discriminationdetection device 76 shown in FIG. 9C can additionally get both thedark-field image and the SE image by the dark-field detector 71 and SEdetector 72 respectively.

The foregoing means to diversify the functions of theenergy-discrimination detection device can be combined in use. In theenergy-discrimination detection device 77 shown in FIG. 10A, theaperture plate 34 is placed below the entrance grid 31 to provide abeam-limiting aperture and the dark-field detector 71 is placed belowthe aperture plate 34 to get the dark-field image. Compared with theenergy-discrimination detection device 77, the energy-discriminationdetection device 78 shown in FIG. 10B additionally uses the apertureplate 36 to provide a beam-limiting aperture for the dark-field detector71. The aperture plate 36 with a plurality of apertures is placed belowthe dark-field detector 71 and perpendicular to the optical axis 3A. Theapertures of the aperture plate 36 have different radial sizes. One ofthe apertures is selected and aligned with optical axis 3A to limit theincident electron beam so that the dark-field detector 71 can detect theelectrons within the desired emission angle spread. The selectedaperture is the beam-limiting aperture for the dark-field detector 71.The aperture plate 36 is made of electrostatic material and preferred tobe set at the potential V₀.

FIG. 11 demonstrates a conventional LVSEM using an energy-discriminationdetection device with the energy filter as shown in FIG. 3A. In FIG. 11,the primary electrons are emitted from the electron source 10 along theoptical axis 101 of the LVSEM and accelerated by the anode 11 so as tohave kinetic energies much higher than the landing energies of theprimary electrons on the upper surface of the specimen. The gun aperture12 cuts off the primary electrons with polar angles larger than aspecific value and thereby limiting the current of the primary electronbeam 1 formed by the primary electrons. Then PE beam 1 is focused by thecondenser lens 13 and passes the beam-limit aperture 18. The size ofbeam-limit aperture 18 and the focusing power of the condenser lens 13are selected to get the desired current value of the PE beam 1thereafter. The magnetic objective lens 5, the retarding electrode 6 andthe specimen 4 forms an electromagnetic-compound retarding objectivelens which focuses the PE beam 1 while decelerating the primaryelectrons thereof so as to get a focused probe with the desired lowlanding energy on the upper surface of the specimen 4. The twodeflectors 14 and 16 together dynamically deflect the PE beam 1 so as tomake the focused probe scan the specimen surface with small off-axisaberrations. The secondary emission beam 2 released by the PE beam 1from the specimen surface 4 can be deflected by the Wien filter 15 to beincident to the conventional detector 17 or the energy-discriminationdetection device 70 with respect to the requirements of applications.The energy-discrimination detection device 70 basically comprises aconventional detector 7 and an energy-filter 3 as shown in FIG. 3A, andcan further comprises one or more of the foregoing means so as to havemultiple functions as mentioned above.

In summary, the present invention provides a method for improvingperformance of an energy filter of reflective type used for a chargedparticle beam. The method employs a beam-adjusting lens on an entranceside of a potential barrier of the energy filter to make the chargedparticle beam become a substantially parallel beam to be incident ontothe potential barrier. Consequently, the energy filter employing thismethod or called as advanced energy filter has a fineenergy-discrimination power within a large emission angle spread and ahigh uniformity of energy-discrimination powers over a large FOV. Usingan advanced energy filter in the energy-discrimination detection deviceof a LVSEM, can obviously improve image contrast. Further, the inventionuses an advanced energy filter to form multiple energy-discriminationdetection devices which can provide variant images, such as SE image,BSE image and dark-field image, of interested features on a specimensurface for multiple application purposes.

Although the present invention has been described in accordance with theembodiments shown, one of ordinary skill in the art will readilyrecognize that there could be variations to the embodiments and thosevariations would be within the spirit and scope of the presentinvention. Accordingly, many modifications may be made by one ofordinary skill in the art without departing from the spirit and scope ofthe appended claims.

What is claimed is:
 1. An energy filter, comprising: a grid electrodebeing set at a first potential to form a potential barrier; and, abeam-adjusting lens being excited to make a charged particle beam becomea substantially parallel beam to be incident onto said potentialbarrier, wherein a first plurality of particles of said charged particlebeam, which has initial kinetic energies higher than a specific valueand thus is able to cross said potential barrier, passes through saidgrid electrode and forms an exiting beam, while a second plurality ofparticles of said charged particle beam, which has initial kineticenergies not higher enough to be able to cross said potential barrier,is reflected back from said grid electrode and forms a reflection beam,wherein an optical axis of said beam-adjusting lens is an optical axisof said energy filter.
 2. The energy filter according to claim 1,wherein said grid electrode is perpendicular to and aligned with saidoptical axis of said beam-adjusting lens.
 3. The energy filter accordingto claim 2, wherein said charged particle beam enters said energy filteralong said optical axis thereof.
 4. The energy filter according to claim2, wherein said charged particle beam enters said energy filter with anangle and a radial shift both with respect to said optical axis thereof.5. The energy filter according to claim 2, wherein said beam-adjustinglens is an electrostatic lens comprising a first electrode, a secondelectrode and a third electrode, wherein with respect to said chargedparticle beam, said first electrode and said second electrode arerespectively on an entrance side and an exit side of said beam-adjustinglens and said third electrode is between said first electrode and saidsecond electrode, wherein said grid electrode is inside said secondelectrode.
 6. The energy filter according to claim 5, wherein saidsecond electrode is set at said first potential, said first electrode isset at a potential of a neighborhood on said entrance side of saidbeam-adjusting lens, and a potential of said third electrode is adjustedto make said charged particle beam become a substantially parallel beamto be incident onto said potential barrier.